1. Field of the Invention
The present invention relates to a method for coating metal parts of a gas turbine with oxidation-resistant and thermal barrier oxides during operation thereof, by adding an additive to a fuel of the gas turbine in order to protect the metal parts exposed to a hot combustion gas from heat during operation.
2. Description of the Related Art
In order to improve thermal efficiency of a system, a gas turbine for aircraft or power generation plants is generally operated at a turbine inlet temperature (TIT) or a combustion temperature of not less than 1,000° C. Under such operational conditions, some parts or components of a gas turbine directly coming into contact with a hot combustion gas are mostly fabricated using nickel-based super-alloys with high thermal resistance. For operation at a high temperature of 1,000° C. or more for a long term, an additional bond-coating (MCrAlY, wherein M is Ni or Co) or bond-coating and zirconia (ZrO2)-based thermal bather coating is applied to a component such as a first stage blade, a vein, a combustion can, and the like in a thickness of above 200 μm.
However, the component such as the first stage blade coming into contact with a hot gas at more than 1,000° C. is fabricated in a complicated form and is subjected to rotary motion, entailing a problem in that the component may not be coated with a thickness of more than 250 μm and, in turn, has restrictions in thermal barrier efficiency (in this case, a thermal bather temperature does not exceed 200° C.). If a thermal carrier coating is stripped or released due to preparation failure of a coating or deterioration of a coating caused by using the same for a long time, metal components covered by the coating are rapidly deteriorated or damaged. Also, a zirconia-based thermal carrier coating cannot effectively shield oxygen contained in a combustion gas and, therefore, the oxygen penetrates through TBC and causes oxidation and deterioration of a bond coating, resulting in release of the coating. In view of coating formation, such a TBC provided during fabrication of gas turbine components requires an additional process, apparatus and/or human labor to apply a coating material to the components and has other drawbacks such as extended period for fabrication of components, high unit cost of production, etc.
An idea or concept that a corrosion-resistant and oxidation-resistant coating is spontaneously formed over a thermal component (such as hot-gas-path component) of a gas turbine by addition of an inorganic or organic compound containing metal ingredients such as silicon to a fuel such as LNG or light oil (diesel) and combustion thereof together with the fuel, has been disclosed in U.S. Pat. No. 4,466,997, entitled “Method of maintaining and repairing protective coatings for the high temperature zones of engines,” EP Laid-Open Application No. 0048910A1, entitled “Protective coatings for the high temperature zones of engines,” EP Laid-Open Application No. 1031546A1, entitled “Method to prevent recession loss of silica and silicon-containing materials in combustion gas environments,” and so forth. According to the foregoing techniques, different metal substances including silicon organic compounds such as silicon, boron, barium, magnesium, calcium titanium compounds, etc., which were added to a fuel, are oxidized using heat of combustion during operation of a gas turbine, and the obtained oxides are applied to a component of an engine coming into contact with a combustion gas in order to protect the existing coating or to regenerate the same (see EP Laid-Open Application No. 0048910A1). EP Laid-Open Application No. 1031546A1 also describes that, if the existing coating (obtained during fabrication of the component) is silica or a surface oxide layer containing silicon of the component is silica, loss of the silica during operation of the gas turbine, which is caused by reaction of the silica with moisture in the combustion gas, is effectively prevented.
The foregoing patent and applications have something in common that an additive is used to enable an oxide coating generated by combustion of a fuel additive to supplement loss of the existing coating. Moreover, these procedures have not disclosed further performance of a new coating obtained using the additive or a material structure of the coating, or physical properties thereof, etc. U.S. Pat. No. 4,466,997 or EP Laid-Open Application No. 0048910A1 have not described exemplary embodiments in relation to formation of a coating for an actual gas turbine. On the other hand, although EP Laid-Open Application No. 1031546A1 included some examples in relation to increase or decrease in weight of a silicon carbide coupon with a size of 1 inch×1 inch×0.5 inch, no measurement or evaluation of a coating formed around an actual gas turbine is disclosed.
Therefore, practical performance and/or functions of an oxide coating containing a fuel additive are unknown by the above conventional technologies. Moreover, the foregoing patent and applications do not disclose specific conditions for formation of a thermal carrier coating. Meanwhile, Korean Patent No. 10-0855703, entitled “Method for formation of a corrosion-resistant and oxidation-resistant coating on a heat resistant component of a gas turbine using a fuel additive,” includes an example for formation of a silica coating over a turbine blade of an actual gas turbine. However, this patent does not describe in detail specific conditions for formation of a coating in association with amount of an additive, evaluation or measurement of corrosion-resistance or oxidation resistance of the formed coating, optimal coating conditions based on the evaluated results, and so forth. In addition, thermal bather performance of the coating is not mentioned therein.